1. Field of the Invention
The present invention relates to a PTC ("positive temperature coefficient of resistance") composite material favorably used in, for example, a current-limiting element which controls fault current.
2. Description of Related Art
The electrical resistance of PTC materials increases sharply with increasing temperature within a particular temperature range. Therefore, PTC materials are used, for example, as current-limiting elements which control fault current in a circuit.
The best known PTC material is a barium titanate type ceramic whose electrical properties change at the Curie point thereof. With this PTC material, however, power loss is large because of its high room temperature resistivity and, moreover, the production cost is high. Hence, the industry began searching for other substances that exhibit the PTC property. As a result, it was discovered that composite materials made of a polymer matrix and a conductive filler exhibited the same type of PTC property possessed by the barium titanate type ceramic.
For example, a mixture consisting of particular proportions of a crystalline polymer (e.g. a polyethylene) as an insulator and conductive particles (e.g. carbon particles) as conductive paths formed in the polymer matrix, exhibits very low room temperature (i.e., 30.degree. C.) electrical resistance, and acts as a conductor as a result of insulator-conductor transition. Specifically, since the polymer has a thermal expansion coefficient that is much greater than that of the conductive particles, the crystalline polymer expands dramatically when the composite material is heated and the crystalline polymer is melted. As a result, the conductive particles forming the conductive paths in the polymer are separated from each other at the melting point temperature of the polymer, the conductive paths are cut, and the electrical resistance of the composite material increases sharply. That is, the composite material exhibits PTC behavior.
When an organic substance such as the above polymer or the like is used as a matrix in a PTC composite material, however, there has been a problem in that when high temperatures caused by fault current continue for a long time, the composite material is unable to exhibit its intended action because the organic substance is generally low in heat resistance. Conventional polymer composite materials also have a problem in that they allow no reliable repeated operation, because the resistance of the material after a trip condition does not return to the initial resistance. It is thus difficult to rely upon these composite materials in sensitive circuit applications.
Studies have also been conducted on composite materials made of a silica type matrix, such as quartz, cristobalite or the like, and conductive particles. Like barium titanate type ceramics, however, these materials are high in room temperature resistivity and thus cause a large power loss. These materials also have a transition temperature (i.e., trip-point temperature) greater than 200.degree. C., thus making their use in certain circuit applications inappropriate.